The present invention relates to a reinforcing patch plate applied to a liquid passage hole formed in a filter-surfaced expressing sheet of an expressing filter press so as to reinforce said liquid passage hole.
Such expressing sheet is made of a flexible fluid-tight material, such as rubber, and, as shown at A in FIG. 1, it is put on a base plate C to cover its opposite surfaces so as to provide an expressing filter plate E having an expressing fluid chamber I. As shown, such filter plates E are arranged alternately with ordinary filter plates G to define prefilt chambers H between adjacent plates. The surface of the expressing sheet A facing said prefilt chamber H is an irregular filter surface J clothed with a filter cloth F, so that when a prefilt is supplied to said prefilt chamber H, the filtrate passing through the filter cloth F will flow into filtrate outlet grooves K of said filter surface J. In order to guide to the outside of the filter press the filtrate flowing into the grooves K, the expressing sheet A is formed in a portion of its filter surface J with a filtrate or liquid passage hole B which communicates with a filtrate outlet passage D open to the margin of the base plate C and also with the filtrate outlet grooves K of the filter surface J. As known in the art, upon completion of the filtration, the expressing fluid chamber I is supplied with an expressing fluid to bulge the expressing sheet A toward the prefilt chamber H (see the left-hand side filter plate E in FIG. 1). Thereupon, the cake remaining in the prefilt chamber H as a result of the filtration is strongly pressed by said expressing sheet A to have its remaining liquid expressed therefrom. The pressing of cake is performed to increase the rate of remaval of liquid as much as possible, and the urging of the expressing sheet against the cake is performed by an expressing fluid under high pressure. The expressing sheet A, therefore, over the filter surface area occupying the greater portion thereof, is urged against the cake with an expressing pressure of about 30 kg/cm.sup.2 up to 100 kg/cm.sup.2. In order to keep fluid-tight the expressing fluid chamber into which a sufficiently high pressure fluid to effect such expression is introduced, the expressing sheet A is clamped between the base plate C and the filter plate G with a pressure greater than said expressing pressure to overcome the high pressure of said expressing fluid and maintain the sheet in close contact with the base plate C.
Under these circumstances, the expressing sheet A is squeezed with a high pressure over its entire area from opposite sides, with the result that the sheet tends to decrease in thickness and to spread out in a plane, undergoing plastic deformation. This deformation is directed to the outer periphery of the expressing sheet A and to the liquid passage hole B where little or no resistance is encountered. The repetition of filtration and expression results in the liquid passage hole B being gradually constricted until it is blocked up. Such constriction or blockage of the liquid passage hole B hinders or prevents the outflow of filtrate. As a result, abnormal high pressure will build up in the filtrate chamber and expressing fluid chamber, entailing the danger of causing damage to such structures as the piping, control and filter plates E. This is also the case with the wash liquor communication passage system for back washing which allows wash liquor to flow from another liquid passage hole for wash liquor (not shown) formed in the expressing sheet to pass through the filter cloth F in a direction reverse to that of the flow of filtrate, namely from the filter surface J of the sheet A toward the prefilt chamber H. In that case, however, since the pressure involved is low, the possible damage would not be so serious.
In an effort to solve the problems described above, there has already been provided a liquid passage hole reinforcing patch plate N, as shown in FIG. 1, which comprises a boss portion M adapted to be fitted in the liquid passage hole B and a liquid guiding flange L disposed at one end of said boss portion to cover the portion of the expressing sheet A around the liquid passage hole B therein (see U.S. Pat. No. 3,988,242). This reinforcing patch plate N is made of a material, such as rubber, which has a hardness equal to or greater than that of the expressing sheet A, and it protects the liquid passage hole B from constriction or blockage due to plastic deformation of the expressing sheet A. However, the reinforcing patch plate N itself is also subjected to said pressure of about 30 kg/cm.sup.2 up to 100 kg/cm.sup.2, with the result that the boss portion M and flange portion L wholly or locally undergo various types of deformation including warp and twist, prematurely losing the function of protecting the liquid passage hole B and the portion of the expressing sheet A therearound. The flange portion L is formed with a plurality of liquid guide grooves O enabling the boss portion M to communicate with the filtrate outlet grooves K formed on the filter surface J of the expressing sheet A. Such grooves O will also be damaged as said various types of deformation take place, thereby hindering the flow of filtrate. Consequently, such reinforcing patch plate N needs frequent replacing, which is troublesome and economically disadvantageous.
The use of metal to make the reinforcing patch plate N would be effective to solve the problem so far as durability is concerned. However, since metal does not agree with the pliability of the expressing sheet A, it locally prevents the expansion and contraction of the expressing sheet A and the related elastic action of the whole, thus giving much fatigue to the boundary between the portion of the expressing sheet A protected by the patch plate N and the unprotected portion therearound. Accordingly, the life of the expressing sheet A itself is greately decreased.